1. Field of the Invention
The present invention relates to methods for amplifying nucleic acids in micro-channels. In some embodiments, the present invention relates to methods for performing a real-time polymerase chain reaction (PCR) in a continuous-flow microfluidic system and to methods for monitoring real-time PCR in such systems.
2. Discussion of the Background
The detection of nucleic acids is central to medicine, forensic science, industrial processing, crop and animal breeding, and many other fields. The ability to detect disease conditions (e.g., cancer), infectious organisms (e.g., HIV), genetic lineage, genetic markers, and the like, may facilitate disease diagnosis and prognosis, marker assisted selection, correct identification of crime scene features, the ability to propagate industrial organisms and many other techniques. Determination of the integrity of a nucleic acid of interest can be relevant to the pathology of an infection or cancer.
One of the most powerful and basic technologies to detect small quantities of nucleic acids is to replicate some or all of a nucleic acid sequence many times, and then analyze the amplification products. PCR is perhaps the most well-known of a number of different amplification techniques.
PCR is a powerful technique for amplifying short sections of DNA. With PCR, one can quickly produce millions of copies of DNA starting from a single template DNA molecule. PCR includes a three phase temperature cycle of denaturation of DNA into single strands, annealing of primers to the denatured strands, and extension of the primers by a thermostable DNA polymerase enzyme. This cycle is repeated so that there are enough copies to be detected and analyzed.
In principle, each cycle of PCR could double the number of copies. In practice, the multiplication achieved after each cycle is always less than 2. Furthermore, as PCR cycling continues, the buildup of amplified DNA products eventually ceases as the concentrations of required reactants diminish.
For general details concerning PCR, see Sambrook and Russell, Molecular Cloning—A Laboratory Manual (3rd Ed.), Vols. 1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (2000); Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., Current Protocols, a joint venture between Greene Publishing Associates, Inc. and John Wiley & Sons, Inc., (supplemented through 2005) and PCR Protocols A Guide to Methods and Applications, M. A. Innis et al., eds., Academic Press Inc. San Diego, Calif. (1990).
Real-time PCR refers to a growing set of techniques in which one measures the buildup of amplified DNA products as the reaction progresses, typically once per PCR cycle. Monitoring the accumulation of products over time allows one to determine the efficiency of the reaction, as well as to estimate the initial concentration of DNA template molecules. For general details concerning real-time PCR see Real-Time PCR: An Essential Guide, K. Edwards et al., eds., Horizon Bioscience, Norwich, U.K. (2004).
Several different real-time detection chemistries now exist to indicate the presence of amplified DNA. Most of these depend upon fluorescence indicators that change properties as a result of the PCR process. Among these detection chemistries are DNA binding dyes (such as SYBR® Green) that increase fluorescence efficiency upon binding to double stranded DNA. Other real-time detection chemistries utilize Foerster resonance energy transfer (FRET), a phenomenon by which the fluorescence efficiency of a dye is strongly dependent on its proximity to another light absorbing moiety or quencher. These dyes and quenchers are typically attached to a DNA sequence-specific probe or primer. Among the FRET-based detection chemistries are hydrolysis probes and conformation probes. Hydrolysis probes (such as the TaqMan® probe) use the polymerase enzyme to cleave a reporter dye molecule from a quencher dye molecule attached to an oligonucleotide probe. Conformation probes (such as molecular beacons) utilize a dye attached to an oligonucleotide, whose fluorescence emission changes upon the conformational change of the oligonucleotide hybridizing to the target DNA.
A number of commercial instruments exist that perform real-time PCR. Examples of available instruments include the Applied Biosystems PRISM 7500, the Bio-Rad iCylcer, and the Roche Diagnostics LightCycler 2.0. The sample containers for these instruments are closed tubes which typically require at least a 10 μl volume of sample solution. If the lowest concentrations of template DNA detectable by a particular assay were on the order of one molecule per microliter, the detection limit for available instruments would be on the order of tens of targets per sample tube. Therefore, in order to achieve single molecule sensitivity, it is desirable to test smaller sample volumes, in the range of 1-1000 nl.
More recently, a number of high throughput approaches to performing PCR and other amplification reactions have been developed, e.g., involving amplification reactions in microfluidic devices, as well as methods for detecting and analyzing amplified nucleic acids in or on the devices. Thermal cycling of the sample for amplification is usually accomplished in one of two methods. In the first method, the sample solution is loaded into the device and the temperature is cycled in time, much like a conventional PCR instrument. In the second method, the sample solution is pumped continuously through spatially varying temperature zones.
For example, Lagally et al. (Anal Chem 73:565-570 (2001)) demonstrated amplification and detection of single template DNA in a 280 nl PCR chamber. Detection of products was made post-PCR using capillary electrophoresis. On the other hand, Kopp et al. (Science 280:1046-1048 (1998)) demonstrated continuous-flow PCR using a glass substrate with a serpentine channel passing over three constant temperature zones at 95° C. (denature), 72° C. (extension), and 60° C. (annealing). The 72° C. zone was located in the central region and had to be passed through briefly in going from 95° C. to 60° C.
Detection was made post-PCR using gel electrophoresis. Since this PCR technique is not based on heating the entire surfaces of the reaction vessel, the reaction rate is determined by a flow rate, not a heating/cooling rate. Neither of these references described real-time monitoring of the PCR reaction.
Park et al. (Anal Chem 75:6029-6033 (2003)) describe a continuous-flow PCR device that uses a polyimide coated fused silica capillary wrapped into a helix around three temperature-controlled blocks. Sample volumes were 2 μl. Detection was made post PCR using gel electrophoresis. Reference was made to the possibility of adapting their instrument for real-time PCR by using a capillary coated with PTFE instead of non-transparent polyimide. See also, Hahn et al. (WO 2005/075683).
Enzelberger et al. (U.S. Pat. No. 6,960,437) describe a microfluidic device that includes a rotary channel having three temperature zones. A number of integrated valves and pumps are used to introduce the sample and to pump it through the zones in a rotary fashion.
Knapp et al. (U.S. Patent Application Publication No. 2005/0042639) describe a microfluidic device capable of single molecule amplification. A planar glass device with several straight parallel channels is disclosed. A mixture of target DNA and PCR reagents is injected into these channels. In a first embodiment, the channels are filled with this mixture and flow is stopped. Then the entire length of the channels is thermally cycled. After thermal cycling is completed, the channels are imaged in order to detect regions of fluorescence where DNA has been amplified. In a second embodiment, the PCR mixture flows continuously through the amplification zone as the temperature is cycled, and fluorescence is detected downstream of the amplification zone. Different degrees of amplification are achieved by altering the time spent in cycling, through changing distance traveled under cycling, and the like. It is worth noting that this method varies conditions (such as cycles experienced) for separate consecutive sample elements, rather than monitoring the progress of individual sample elements over time.
A need exists for robust, high throughput methods of real-time PCR that can be performed efficiently and accurately.